Carbon nanostructure takes infinite shape

They call this new compound infinitene: 12 benzene rings fused together to make a figure eight or an infinity symbol, depending on your point of view.

The molecule was made at Nagoya University by Maciej Krzeszewski, Hideto Ito, and Kenichiro Itami, who are experts in building new carbon-based macromolecules with interesting structures (ChemRxiv 2021, DOI: 10.33774/chemrxiv-2021-pcwcc).

Looped polyaromatic compounds have fascinated chemists for years. As well as posing a synthetic challenge, these compounds can help researchers explore the limits of aromaticity.

To synthesize the new molecule, the team took inspiration from the synthesis of kekulene, a planar ring of 12 fused benzene rings that chemists first made nearly 40 years ago (Chem. Ber. 1983, DOI: 10.1002/cber.19831161021). The group also drew from a previous attempt to make infinitene (Bull. Chem. Soc. Jpn. 2000, DOI: 10.1246/bcsj.73.185). That foundational work helped them make the compound, otherwise called cyclo[c.c.c.c.c.c.e.e.e.e.e.e]dodecakisbenzene, before characterizing its properties.

The team confirmed the structure of their new molecule using X-ray crystallography, which showed that only 3.192 Å separates the upper and lower benzene rings where the molecule crosses over itself.

The crystal structure also allowed the researchers to calculate the electron distribution within the compound’s molecular orbitals. These calculations confirmed that its π-electrons are delocalized within individual benzene rings rather than being delocalized across the entire molecule.

That was what the team expected, Itami says. This behavior is also observed in many other helicenes, including kekulene.

Itami says the synthetic challenge of making infinitene was what inspired his team, but the shape was also a draw.

Infinitene’s shape “is fascinating and beautiful,” agrees Shinji Toyota at Tokyo Institute of Technology. He says the characterization data will also be valuable for other groups developing optical or additional materials from aromatic units. The molecule, he says, shows the “infinite possibilities of molecular design of hydrocarbon chemistry.”